1. Field of the Invention
The present invention relates generally to implantable cardiac stimulation devices. The present invention more particularly relates to methods, systems and devices for adjusting cardiac pacing parameters to optimize pacing effectiveness.
2. Related Art
Implantable cardiac stimulation devices are well known in the art. Such devices may include, for example, implantable cardiac pacemakers and defibrillators either alone or combined in a common enclosure. The devices are generally implanted in a pectoral region of the chest beneath the skin of a patient within what is known as a subcutaneous pocket. The implantable devices generally function in association with one or more electrode carrying leads which are implanted within the heart. The electrodes are positioned within the heart for making electrical contact with the muscle tissue of their respective heart chamber. Conductors within the leads couple the electrodes to the device to enable the device to deliver the desired electrical therapy.
Traditionally, therapy delivery has been limited to the right side of the heart. However, new lead structures and methods have been produced and practiced for also delivering cardiac rhythm management therapy from or to the left heart. These lead structures and methods provide electrode electrical contact with the left atrium and left ventricle of the heart by lead implantation within the coronary sinus of the heart. As is well known, the coronary sinus passes closely adjacent the left atrium, extends into the great vein adjacent the left ventricle, and then continues adjacent the left ventricle towards the apex of the heart.
It has been demonstrated that electrodes placed in the coronary sinus and great vein may be used for left atrial pacing, left ventricular pacing, and cardioversion and defibrillation. These advancements enable implantable cardiac stimulation devices to address the needs of the wide patient population, from those that would benefit from right heart side pacing alone, to those that would benefit from left heart side pacing in conjunction with right heart side pacing (bi-chamber pacing), to those that would benefit from left heart side pacing alone.
The potential of multi-site pacing to improve the hemodynamic status (also referred to as cardiac performance) of select patient populations is well established in the research community. One area of active research is in determining optimal ventricular pacing configurations. For example, the results of one study suggest that optimal results are obtained by pacing on the side of the heart that has the conduction delay, so that left ventricular pacing give superior performance for patients with a left bundle branch block, while right ventricular pacing yields better results in patients with right bundle branch block. As illustrated by those who conducted this study, the problem is typically couched in terms of pacing mode, so that comparison is made among right ventricular pacing, left ventricular pacing, and simultaneous bi-ventricular pacing. Unfortunately, this approach considers only a small subset of the parameter space, and therefore carries the risk of missing altogether the optimal pacing configuration (also referred to as the optimum pacing parameters).
Thus, there exists the further challenge, with multi-site pacing, of identifying the optimal pacing configuration (i.e., determining optimal pacing parameters). This challenge is complicated by the fact that only a limited region of the left ventricle is accessible for pacing, particularly when access is obtained via the coronary venous system.
A further challenge in multi-site pacing is that the number of potential pacing parameter combinations increases rapidly as the number of sites paced increases, making it that much more difficult to determine optimal pacing parameters. This problem can be formulated in terms of an abstract mathematical space, where each dimension of the space represents an adjustable independent pacing parameter. For example, in dual-chamber pacing (e.g., right atrium and right ventricle pacing) there is a single independent parameter, the atrioventricular (AV) delay (assuming a fixed pacing rate), and cardiac performance is a function of this parameter. In three chamber pacing (e.g., right atrium, right ventricle and left ventricle) there are two independent parameters (AV delay and interventricular RV-LV delay), and thus a two dimensional parameter space (assuming a fixed pacing rate). This can be generalized to any number of dimensions. For example, the parameter space of a four-chamber pacemaker would have three dimensions (assuming a fixed pacing rate). If pacing rate is also being analyzed then another dimension is added.
Although not efficient, an exhaustive search of a one dimensional parameter space (e.g., when two-sites are being paced with a fixed pacing rate) is possible, as shown in U.S. Pat. No. 5,540,727. However, as the number of sites (and thus, parameters) increases, an exhaustive search of parameter space quickly become unworkable, as illustrated in the following table.
## of# of test pointstime averagetotal time for anof sitesdimensionsper dimensionper test pointexhaustive search21105 sec50 sec32105 sec8 min43105 sec1 hour, 20 min
Accordingly, there is a need for more efficient and effective methods, systems and devices for optimizing multiple pacing parameters.
An additional challenge in multi-site pacing is that the optimal pacing configuration is dependent on the physiologic state of the patient. In patients with Hypertrophic Obstructive Cardiomyopathy, for example, the degree of obstruction is dependent on posture. Thus, the optimal pacing configuration for an unseated, walking patient is likely to be different from what is optimal for a patient who is sedated and supine on the examination or operating table. Thus, there is a need for efficient and effective methods, systems and devices for optimizing pacing parameters, that can dynamically adapt to changes in the physiologic state of the patient.
Further, the optimal pacing configuration may change as the patient's myocardial state changes. Myocardial remodeling is associated with the progression or regression of heart failure. Such remodeling may depend on response to therapy, lifestyle changes, and age. As the heart remodels, the optimum sequence of activation may change. For example, in the acute phase of pacemaker implantation, left ventricular pacing may have been optimal for a given patient. Over weeks or months, the heart may remodel such that more synchronous bi-ventricular pacing becomes optimal. Thus, there is a need for efficient and effective methods, systems and devices for optimizing pacing parameters, that can dynamically adapt to changes in the myocardial state of the patient.